The present invention relates to an apparatus for detecting misfire occurring in a multicylinder internal combustion engine on the basis of the operating condition of the internal combustion engine.
When misfire occurs in an internal combustion engine, complete combustion in a combustion chamber of the internal combustion engine is not achieved, and consequently the engine rotational speed decreases. A prior art apparatus for detecting misfire in a multicylinder engine based on this phenomenon has been known, as is disclosed in JP-A-61-258955, for example, which detects a momentary rotational speed at least at two points in each expansion stroke of each engine cylinder, detects engine speed fluctuations in each expansion stroke of each cylinder, obtains a change .DELTA..omega. in the engine speed fluctuations (hereinafter simply referred to as "a speed variation quantity .DELTA..omega.") from the engine speed fluctuations which have been detected in the current and last expansion strokes, compares the speed variation quantity .DELTA..omega. with a predetermined reference misfire determination value, and determines that misfire has occurred in the associated cylinder, if the speed variation quantity is greater than the reference misfire determination value.
However, in such a prior art misfire detecting apparatus, occurrence of misfire in an internal combustion engine is determined based upon a misfire determination value which is uniformly set for all cylinders. Therefore, if the combustion state varies for respective cylinders and the speed variation quantity .DELTA..omega. varies for respective cylinders, there is a danger of determining erroneously that misfire has occurred in a cylinder exhibiting a large speed variation quantity .DELTA..omega., in spite that the cylinder is in a normal operating condition and no misfire has occurred. The above-mentioned speed variation quantity .DELTA..omega. for each cylinder increases as the engine rotational speed increases, because vibration of the body of the internal combustion engine also increases.
FIG. 11 shows a result of measurement of the speed variation quantity .DELTA..omega. when a six-cylinder internal combustion engine used in a vehicle is operated at a high rotational speed of 4,500 r.p.m. and the vehicle runs on a flat road. The speed variation quantity .DELTA..omega. varies greatly for respective cylinders as shown by a solid line in the drawing, even if no misfire occurs in the internal combustion engine. The speed variation quantity .DELTA..omega.of a fourth cylinder #4 becomes greater than that in a normal operation state as shown by a dotted line in FIG. 11 when misfire occurs in the fourth cylinder #4. Accordingly, a fire determination value may be set between the speed variation quantity .DELTA..omega. at the time of occurrence of misfire in the fourth cylinder #4 and the speed variation quantity .DELTA..omega. in a normal operation state of the fourth cylinder #4. However, if misfire determination of a third cylinder #3 is made by the use of the same misfire determination value as that of the fourth cylinder #4, it is determined erroneously that misfire has also occurred in the third cylinder #3, because the speed variation quantity .DELTA..omega. of the third cylinder #3, even when no misfire is occurring therein, is greater than the speed variation quantity .DELTA..omega. of the fourth cylinder #4. Thus, it is difficult to perform accurate misfire detection only for the actually misfiring cylinder #4.
Accordingly, if a misfire detection value is uniformly set for all cylinders, erroneous determination may be made such that misfire has occurred in a cylinder having a large speed variation quantity .DELTA..omega., even it is in a normal operation state, as is the case with the third cylinder #3 shown in FIG. 11. Furthermore, since the speed variation quantity .DELTA..omega. varies greatly at a high rotational speed of the engine, the misfire detection accuracy becomes worse in such a high speed operating state.
The above-mentioned problem can be solved by preliminarily setting a misfire determination value for each cylinder. In this case, the misfire determination values for respective cylinders have to be stored. In a multicylinder internal combustion engine having, for example, 8 or 12 cylinders, excessively large storage capacity is required for storing all the misfire determination values.